Methods and systems for combined cyclic delay diversity and precoding of radio signals

ABSTRACT

In a transmitter or transceiver, signals can be precoded by multiplying symbol vectors with various matrices. For example, symbol vectors can be multiplied with a first column subset of unitary matrix which spreads symbols in the symbol vectors across virtual transmit antennas, a second diagonal matrix which changes a phase of the virtual transmit antennas, and a third precoding matrix which distributes the transmission across the transmit antennas.

RELATED APPLICATION

The present application is related to, and claims priority from, SwedishPatent Application No. 0700367-6, filed on Feb. 13, 2007, the disclosureof which is incorporated here by reference.

TECHNICAL FIELD

The present invention generally relates to radio communication systems,devices, software and methods and, more particularly, to mechanisms andtechniques for combining precoding and cyclic delay diversity associatedtherewith.

BACKGROUND

At its inception radio telephony was designed, and used for, voicecommunications. As the consumer electronics industry continued tomature, and the capabilities of processors increased, more devicesbecame available use that allowed the wireless transfer of data betweendevices and more applications became available that operated based onsuch transferred data. Of particular note are the Internet and localarea networks (LANs). These two innovations allowed multiple users andmultiple devices to communicate and exchange data between differentdevices and device types. With the advent of these devices andcapabilities, users (both business and residential) found the need totransmit data, as well as voice, from mobile locations.

The infrastructure and networks which support this voice and datatransfer have likewise evolved. Limited data applications, such as textmessaging, were introduced into the so-called “2G” systems, such as theGlobal System for Mobile (GSM) communications. Packet data over radiocommunication systems became more usable in GSM with the addition of theGeneral Packet Radio Services (GPRS). 3G systems and, then, even higherbandwidth radio communications introduced by Universal Terrestrial RadioAccess (UTRA) standards made applications like surfing the web moreeasily accessible to millions of users (and with more tolerable delay).

Even as new network designs are rolled out by network manufacturers,future systems which provide greater data throughputs to end userdevices are under discussion and development. For example, the so-called3GPP Long Term Evolution (LTE) standardization project is intended toprovide a technical basis for radiocommunications in the decades tocome. Among other things of note with regard to LTE systems is that theywill provide for downlink communications (i.e., the transmissiondirection from the network to the mobile terminal) using orthogonalfrequency division multiplexing (OFDM) as a transmission format and willprovide for uplink communications (i.e., the transmission direction fromthe mobile terminal to the network) using single carrier frequencydivision multiple access (FDMA).

Another interesting feature of LTE is its support for multiple antennasat both the transmit side and the receive side. This provides theopportunity to leverage several different techniques to improve thequality and/or data rate of received radio signals. Such techniquesinclude, for example, diversity against fading (e.g., spatialdiversity), shaping the overall antenna beam to maximize gain in thedirection of the target (beamforming), and the generation of what can beseen as multiple, parallel “channels” to improve bandwidth utilization(spatial multiplexing or multi-input multi-output (MIMO).

Precoding is a popular technique used in conjunction with multi-antennatransmission. The basic principle involved in precoding is to mix anddistribute the modulation symbols over the antennas while potentiallyalso taking the current channel conditions into account. Precoding canbe implemented by, for example, multiplying the information carryingsymbol vector containing modulation symbols by a matrix which isselected to match the channel. Sequences of symbol vectors thus form aset of parallel symbol streams and each such symbol stream is referredto as a “layer”. Thus, depending on the choice of precoder in aparticular implementation, a layer may directly correspond to a certainantenna or a layer may, via the precoder mapping, be distributed ontoseveral antennas.

Cyclic delay diversity (CDD) is a form of open-loop precoding in whichthe precoding matrix is intentionally varied over the frequency withinthe transmission (or system) bandwidth. Typically, this is realized byintroducing different cyclic time delay for the different antennas, oralternatively realized by varying the phase of the transmitted signalsfrom the different antennas. This kind of phase shift means that theeffective channel, comprising the true channel and the CDD precoding,varies faster over frequency than the original channel. By distributingthe transmission over frequency, this kind of artificially inducedfrequency-selectivity is useful in achieving frequency diversity.

One of the more significant characteristics of the radio channelconditions to consider in the context of high rate, multi-antennatransmission is the so-called channel rank. Generally speaking, thechannel rank can vary from one up to the minimum of number of transmitand receive antennas. For example, given a 4×2 system as an example,i.e., a system with four transmit antennas and two receive antennas, themaximum channel rank is two. The channel rank associated with aparticular connection varies in time and frequency as the fast fadingalters the channel coefficients. Moreover, the channel rank determineshow many layers, also referred to as the transmission rank, can besuccessfully transmitted simultaneously. For example, if the channelrank is one at the instant of the transmission of two layers, there is astrong likelihood that the two signals corresponding to the two layerswill interfere so much that both of the layers are erroneously detectedat the receiver. In conjunction with precoding, adapting thetransmission to the channel rank involves striving for using as manylayers as the channel rank.

FIG. 1 illustrates a transmission structure 108 for combining CDD and,possibly channel dependent, precoding. Therein, each layer 110 createdby the transmitter presents a stream of information carrying modulationsymbols to the CDD based precoder 112 as a sequence of symbol vectors114. The CDD precoder 112 applies the two matrices 116 and 118illustrated therein to each incoming symbol vector to perform theprecoding process. More specifically, the CDD precoder 112 first appliesthe matrix U_(N) _(T) _(×r) 118 to the symbol vector 114, followed bythe diagonal CDD matrix 116. U_(N) _(T) _(×r) matrix 118 is a columnsubset of a (possibly scaled) unitary matrix, r denotes the transmissionrank and N_(T) is the number of transmit antennas in the transmittingdevice. The notation

_(k×l) means a matrix

 having k rows and l columns. The diagonal CDD matrix 116 has non-zerovalues along the diagonal including an antenna phase shift value Θindexed by a parameter k which may be a function of frequency. If OFDMis used for the transmission, k may e.g. represent the subcarrier indexor the closely related data resource element index (which excludesresource elements containing reference symbols). It should also be notedthat k may be a more arbitrary function of the position of the resourceelements on the resource grid in OFDM. The resulting, precodedmodulation symbol vector is then output for, e.g., resource mapping andOFDM modulation 120, prior to being transmitted via antennas 122 (alsoreferred to as antenna ports).

The transmission structure 108 illustrated in FIG. 1 can be utilized inseveral ways. For example, one option is to use a fixed, channelindependent, unitary matrix U_(N) _(T) _(×r) 118 with a certain numberof columns r corresponding to the transmission rank. The unitary matrix118 serves to distribute each symbol on all antennas 122, while thediagonal CDD matrix 116 varies (shifts) the phase of each antenna 122.This increases the frequency selectivity of the effective channel eachlayer 110 experiences which, as mentioned above, can be useful forachieving frequency diversity (as well as multi-user diversity whenfrequency domain scheduling is used).

There are, however, certain problems associated with using thetransmission structure 108 illustrated in FIG. 1 to perform precoding.The spatial correlation properties vary as a function of k and thesevariations need to be fast in order to ensure sufficient frequencydiversity over even rather narrowband transmissions. This makes itdifficult for a receiver to estimate the properties of interferencestemming from such kind of transmissions. The transmission structure 108also does not provide sufficient freedom to design the precoding ontothe antenna ports. Furthermore, considering for example a r=1 rank onetransmission, the transmission structure 108 will inherently use onecolumn of the U_(N) _(T) _(×r) matrix 118 to apply to the incomingsymbol vector 114. This column would for example (in a two transmitantenna scenario) be equal to [1, 1]. Thus, this column together withthe diagonal CDD matrix 116, forms a frequency selective beamformerwhich may be varied in a periodic fashion over the scheduled bandwidth.The period will depend on the selected speed of the phase variations.However, such beamforming may be problematic because, if the MIMOchannel is correlated at the transmit side, severe cancellation ofsignals may occur at some frequencies. If the coding rate is not lowenough over the scheduled bandwidth, this will in turn result incommunication errors. Similar cancellation can occur even fortransmission ranks greater than one. So, generally, it will be difficultto use high coding rates in conjunction with the transmission structure108 (and its technique for precoding) if the scheduled bandwidth is overa significant portion of the previously mentioned beamformer period.Such a scenario, however, typically occurs when large delay CDD is used,i.e., corresponding to fast phase shift variations in the frequencydomain.

Accordingly, it would be desirable to provide precoding systems,methods, devices and software which avoid the afore-described problemsand drawbacks.

SUMMARY

According to one exemplary embodiment, a method for transmittinginformation signals having a plurality of symbol vectors associatedtherewith on a radio channel includes precoding the symbol vectors bymultiplying the symbol vectors with: a first column subset of a unitarymatrix which spreads symbols in the symbol vectors across all virtualtransmit antennas, a second diagonal matrix which changes a phase of thevirtual transmit antennas, and a third precoding matrix whichdistributes transmit energy across physical transmit antennas, furtherprocessing the precoded symbol vectors to generate the informationsignals, and transmitting the information signals.

According to another exemplary embodiment, a transmitter fortransmitting information signals having a plurality of symbol vectorsassociated therewith on a radio channel includes: a plurality ofphysical transmit antennas, a processor for precoding the symbol vectorsby multiplying the symbol vectors with: a first column subset of aunitary matrix which spreads symbols in the symbol vectors across allvirtual transmit antennas, a second diagonal matrix which changes aphase of the virtual transmit antennas, and a third precoding matrixwhich distributes transmit energy across the physical transmit antennas,and for further processing the precoded symbol vectors to generate theinformation signals; and a transmit chain of elements for transmittingthe information signals.

According to another exemplary embodiment, a method for equalizingreceived information signals having a plurality of symbol vectorsassociated therewith includes forming a channel estimate associated withthe received information signals by multiplying an initial channelestimate with a plurality of matrices, the plurality of matricesincluding: a first column subset of a unitary matrix, a second diagonalmatrix, and a third precoding matrix, and equalizing the informationsignals using the formed channel estimate.

According to another exemplary embodiment, a processor forms a channelestimate associated with received information signals by multiplying aninitial channel estimate with a plurality of matrices, the plurality ofmatrices including: a first column subset of a unitary matrix, a seconddiagonal matrix, and a third precoding matrix, and wherein the processoruses the formed channel estimate to equalize the received informationsignals.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 illustrates a transmission structure including a conventionalprecoder;

FIG. 2 illustrates an exemplary LTE access network in which exemplaryembodiments can be implemented;

FIG. 3 depicts exemplary LTE physical layer information signalprocessing with which exemplary embodiments can be associated;

FIG. 4 shows an example of an antenna mapping function in more detail;

FIG. 5 illustrates a transmission structure including a precoderaccording to an exemplary embodiment;

FIG. 6 is a block diagram of an exemplary transmitting device in whichprecoding according to these exemplary embodiments can be implemented;

FIG. 7 is a flowchart illustrating a method for transmitting informationsignals according to an exemplary embodiment;

FIG. 8 is a block diagram of an exemplary receiving device in whichsignals which have been precoded according to these exemplaryembodiments can be processed; and

FIG. 9 is a flowchart illustrating a method for processing receivedinformation signals according to an exemplary embodiment.

DETAILED DESCRIPTION

The following description of the exemplary embodiments of the presentinvention refers to the accompanying drawings. The same referencenumbers in different drawings identify the same or similar elements. Thefollowing detailed description does not limit the invention. Instead,the scope of the invention is defined by the appended claims.

As mentioned above, the transmission structure 108 illustrated in FIG. 1and, more particularly, the CDD precoder 112, suffer from certaindrawbacks when considering its applicability in the context of matrices118 which are channel independent. In addition to the problem describedin the Background section, another problem with the transmissionstructure 108 can occur if channel dependent precoding is to be used inconjunction with CDD. Since the diagonal CDD matrix 116 is applied tothe symbol vector 114 before the, in this example, channel dependentprecoding matrix 118, the precoding matrix 118 will then need to dealwith a more frequency-selective effective channel, i.e., comprising thetrue channel and the applied CDD diagonal matrix 118. In order to ensurean effective precoding scheme under these circumstances, the precoder112 must then switch the elements representing matrix 118 at a finerfrequency granularity than if only the original channel was present.This, in turn, may lead to substantially higher signaling overheadbecause the precoder elements which are used to precode transmittedsymbols are typically identified to the receiver in the form additional(overhead) signaling.

According to exemplary embodiments these problems are addressed byproviding a different transmission structure in which, for example, anadditional (channel dependent or channel independent) precoder elementis applied to the symbol vector output from the CDD operation comprisingapplication of diagonal CDD matrix and column subset of a channelindependent unitary matrix as described above. This can be seen by, forexample, noting the additional matrix 515 in FIG. 5, where for futurereference, it should be noted that the symbol vector after applicationof the diagonal CDD matrix is referred to as virtual antennas. Theresulting vector x(k) transmitted on a resource indexed by k can thus bewritten asx(k)=W _(N) _(T) _(×l)(k)D(k)U _(l×r) s(k)  (1)where D(k) is the second diagonal CDD matrix 516 and it is emphasizedthat the third precoding matrix 515, W_(N) _(T) _(×l)(k), maypotentially be different for different values of k. The parameter lwould here typically be set to equal the transmission rank r. Theseexemplary embodiments can be used to, for example, add a channeldependent precoding stage directly at the input of the true channel(i.e., outputting onto the antenna ports), which in turn allows CDD tobe combined with channel dependent precoding without requiring finerprecoding granularity, thus saving signaling overhead. Even if the thirdprecoding matrix is not channel dependent, the structure indicated bythe exemplary embodiments provides additional freedom in selectingsuitable precoders for the third precoding stage so as to avoid some ofthe previously mentioned problems associated with the use of thestructure 108.

To provide some context for the more detailed discussion of combined CDDand precoding according to these exemplary embodiments, consider firstthe exemplary radiocommunication system illustrated in FIGS. 2-4.Beginning with the radio access network nodes and interfaces in FIG. 2,it will be seen that this particular example is provided in the contextof LTE systems. Nonetheless, the present invention is not limited in itsapplicability to transmitters or transmissions associated with LTEsystems and can instead be used in any system wherein multiple transmitantennas and precoding are employed, including, but not limited toWideband Code Division Multiple Access (WCDMA), GSM, UTRA, E-UTRA, HighSpeed Packet Access (HSPA), UMB, WiMaX and other, systems, devices andmethods. Since, however, the example in FIG. 2 is provided in terms ofLTE, the network node which transmits and receives over the airinterface is termed an eNodeB, several of which eNodeBs 200 areillustrated therein.

In the context of the air interface, each eNodeB 200 is responsible fortransmitting signals toward, and receiving signals from, one or morecells 202. Each eNodeB includes multiple antennas, e.g., 2, 4, or moretransmit antennas, and handles functions including, but not limited tocoding, decoding, modulation, demodulation, interleaving,de-interleaving, etc., with respect to the physical layer of suchsignals. Note that the phrase “transmit antennas” as used herein isspecifically meant to include, and be generic to, physical antennas,virtual antennas and antenna ports. The eNodeBs 200 are also responsiblefor many higher functions associated with handling communications in thesystem including, for example, scheduling users, handover decisions, andthe like. The interested reader who desires more information regardingtransmit or receive functions associated with LTE or other systems inwhich these exemplary embodiments may be deployed is directed toward thebook entitled “3G Evolution—HSPA and LTE for Mobile Broadband”, to ErikDahlman et al., published by Elsevier Ltd., 2007, the disclosure ofwhich is incorporated by reference.

Nonetheless, to briefly discuss the baseband processing associated withthe transmission of signals in the downlink (i.e., possibly transferredthrough the core network 203 to an eNodeB 200 and then into the cells202 toward target mobile terminal or stations, e.g., MS 204 in FIG. 2),consider FIG. 3. Therein, two transport blocks of data 300 are beingprocessed for transmission by an eNodeB 200 using spatial multiplexing.Cyclic redundancy check (CRC) bits are inserted at steps 302 to be usedby the receiver to detect errors. Channel coding is applied to thetransport blocks at steps 304 to provide protection to the payload dataagainst the impairments presented by the radio channel. The hybridautomatic retransmission request (HARQ) steps 306 operate to extract orrepeat code bits from the blocks of code bits provided by the channelencoder to generate a precise set of bits to be transmitted within atransmit time interval (TTI), e.g., based upon various criteria such asthe number of assigned resource blocks, the selected modulation schemeand the spatial multiplexing order.

At step 308, the code words output from the HARQ block are scrambled(multiplied) by a bit-level scrambling sequence or mask, which aids thereceive in suppressing interference to the radio signal. The selecteddata modulation, e.g., Quadrature Phase-Shift Keying (QPSK), 16Quadrature Amplitude Modulation (QAM), or 64 QAM, is then applied atstep 310 to transform blocks of scrambled bits into corresponding blocksof modulation symbols. These modulation symbols are then mapped todifferent antennas and/or different antenna ports at step 312. In LTEnomenclature, an antenna port corresponds to the transmission of aparticular downlink reference signal which may, or may not, correspondto an actual, physical antenna. The symbols to be transmitted on eachantenna (1-n in FIG. 3, e.g., 2, 4, 8, 16) are then mapped to respectiveresource blocks 314 and sent off for OFDM processing (not shown) priorto transmission by the eNodeB 200.

Of particular interest in the downlink processing for these exemplaryembodiments is the antenna mapping step/block 312. The antenna mappingprocess can be further subdivided into layer mapping of the codewordsoutput from the modulation block 310 and precoding of the resultingsymbol vectors to generate the antenna (or antenna port) mapped symbols,as shown in FIG. 4. Therein an example is provided with two sets ofcodewords being mapped by layer mapping function 400 into three layers.Two symbol vectors v1 and v2 associated with the three layers areillustrated in FIG. 4. These symbol vectors are then precoded byapplying one or more precoding matrices by precoding function 402, i.e.,by matrix multiplication of the precoding matrix or matrices with theincoming symbol vectors. According to one exemplary embodiment, theprecoding function 402 can apply three different matrices as will bedescribed below with respect to FIG. 5. It will be appreciated that theselections of mapping to three layers and four antennas in FIG. 4 ispurely exemplary. Selection of the number of layers will, as describedearlier, vary based upon the channel rank (among possibly othercriteria) and the number of antennas may vary from system to system oreven among transmit devices within systems.

FIG. 5 illustrates a precoder according to exemplary embodiments whichcan be used to perform precoding, e.g., as described with respect toblocks 312 and 402 above. Therein, each layer 510 created by thetransmitter presents a stream of modulation symbols to the CDD basedprecoder 512 as a sequence of symbol vectors 514. The CDD precoder 512applies the three matrices 515, 516 and 518 illustrated therein to eachincoming symbol vector to perform the precoding process. Morespecifically, the CDD precoder 512 according to this exemplaryembodiment first applies the matrix U_(l×r) 518, which is a columnsubset of a possibly scaled unitary l×l matrix, to the symbol vector514, followed by diagonal CDD matrix 516, followed then by a precodingmatrix W_(N) _(T) _(×l) 515 resulting in the transmit vector previouslygiven in equation (1).

The columns of the matrix 518 are taken from a possibly scaled unitarymatrix. A unitary matrix exhibits the property that its inverse is equalto the complex conjugate transpose of the unitary matrix of interest.Thus, the columns of the matrix 518 are orthogonal and of equal norm.The first applied, matrix 518 operates to spread the symbols across theantenna ports. The second applied CDD matrix 516 will have the qualitiesof a diagonal matrix, i.e., elements on one diagonal are non-zero andthe remaining matrix elements are zero. This CDD matrix 516 operates tovary (shift) the phase of each antenna or antenna port 522. The thirdapplied, precoding matrix 515 operates to distribute the transmissionenergy across the antennas or antenna ports. It may be determined ineither a channel independent manner or based upon, at least in part,current radio channel conditions resulting in a channel dependentprecoder operation. As discussed above, application of these matrices tothe incoming symbol vectors can be performed by a processing unit withinthe transmitter by way of matrix multiplication.

The parameter l is introduced in this exemplary embodiment as a sizeparameter of the three matrices used to perform precoding, i.e., thenumber of columns in the last applied precoding matrix 515, the numberof rows and columns in the second applied, diagonal CDD matrix 516 andthe number of rows in the first applied, unitary matrix 518. Thus,unlike the transmission structure illustrated in FIG. 1, the size of thematrices involved in performing precoding according to these exemplaryembodiments may vary dynamically for a given transmitter according tothe transmission rank of the channel (or the number of layers), e.g.,the number of rows in the unitary matrix 518 may be different than thenumber of transmit antennas. As previously mentioned, the parameter l istypically set equal to the transmission rank r. By way of contrast, thematrices 116 and 118 discussed above with respect to FIG. 1 were fixedlysized to the number of transmit antennas associated with the particulartransmitter performing the precoding.

Looking more closely at the three matrices used to perform precodingaccording to this exemplary embodiment shown in FIG. 5, the matrixU_(l×r) 518 is, like matrix 118, a column subset of a (possibly scaled)unitary matrix where l denotes the number of rows in the matrix and rdenotes the transmission rank and number of columns. The diagonal CDDmatrix 516 includes exp(jθ_(n)k) elements along the diagonal whereinθ_(n) represents a phase value associated with a particular antenna orantenna port and k is an index associated with a particular resourceelement (e.g. indices of all subcarriers or indices of only thosesubcarriers which carry data rather than those which carry referencesymbols). The matrix W_(N) _(T) _(×l) 515 is a precoding matrix whichcan have various values, e.g., to perform channel dependent beamformingor precoding in a channel independent manner, some examples of which aredescribed below, and which has a size of N_(T) (the number of transmitantennas/antenna ports in the transmitting device) by l. The resulting,precoded modulation symbol vector is then output for, e.g., resourcemapping and OFDM modulation 520, prior to being transmitted via antennas522.

According to theses exemplary embodiments, the precoding matrix 515W_(N) _(T) _(×l) is now applied directly on the MIMO channel matrix.This means that, in for example the case of a channel-dependentprecoding, W_(N) _(T) _(×l) can “see” the true channel which isunaffected by any potential CDD operation. The diagonal CDD matrix andU_(l×r) can then be used to perform CDD operation on the new, improved,effective channel comprising the true channel and W_(N) _(T) _(×l). Thenumber of rows l can moreover be adapted so that CDD operation is onlyperformed among the virtual antennas taken as input to W_(N) _(T) _(×l).

For example, for transmission rank one (and therefore, the number oflayers is one given that these examples are concerned with spatialmultiplexing), l could be set to one, the diagonal matrix would be 1,and W_(N) _(T) _(×l) would be one column vector performing possiblychannel dependent beamforming. If different W_(N) _(T) _(×l) are usedfor different indices k frequency dependent precoding is possible.Similarly, for transmission rank two, l could be two, U_(l×r) could havetwo columns, and W_(N) _(T) _(×l) could be channel dependent and havetwo columns as well. The diagonal matrix together with U_(l×r) thenperforms CDD like operation on the virtual antennas, meaning that thetwo layers see a mixture of the virtual antenna channels which in turnare formed from the true channel and W_(N) _(T) _(×l). Thus, the threematrices 515, 516 and 518 could, for example, be selected from thefollowing table:

Maximum Number of Layers W D U 1 $\quad\begin{bmatrix}a \\b\end{bmatrix}$ [1] [1] 2 $\quad\begin{bmatrix}c & d \\e & f\end{bmatrix}$ $\quad\begin{bmatrix}g & 0 \\0 & h\end{bmatrix}$ $\quad\begin{bmatrix}k & l \\m & n\end{bmatrix}$Therein, variables a, b, c, d, e, f, g, h, k, l, m, and n represent,potentially complex, values which are selected to provide the functionsor matrix-types described above, resulting in a so-called precodercodebook. Examples of these values can be found in, for example, thestandards specification 3GPP TS 36.211 V1.3.1, (2007-08), at section6.3.3.2. By using precoding as described above and illustrated in FIG.5, beamforming on the virtual antennas (antenna ports) spreads energy indesignated sub-spaces, which sub-spaces focus the transmission energytoward the intended recipient (e.g., mobile station) of thetransmission. Channel independent precoding is also possible in forexample the sense of varying the selection of precoders in a more randommanner so as to avoid focusing the energy in any particular direction.

According to another exemplary embodiment, a transmission structurecould provide for precoding wherein the precoding matrix 515 i.e., W_(N)_(T) _(×l), is instead set to be a fixed channel and frequencyindependent matrix with orthogonal and equal norm columns, the diagonalCDD matrix set to be of size N_(T)×N_(T) (i.e., a square matrix equal tothe number of transmit antennas) and the matrix 518 U_(l×r) can then bea single column of all ones. This exemplary embodiment provides anotherform of CDD which does not suffer from the previously mentionedcancellation problem when correlated fading is present on the transmitside.

As mentioned above, the transmit processing techniques described hereinmay be used for various communication systems such as Code DivisionMultiple Access (CDMA) systems, Time Division Multiple Access (TDMA)systems, Frequency Division Multiple Access (FDMA) systems, OrthogonalFDMA (OFDMA) systems, Single-Carrier FDMA (SC-FDMA) systems, etc. Thetransmitter may, for example, be disposed within a radio base station,NodeB, eNodeB, or the like, to transmit information signals on adownlink radio channel. Alternatively, the transmitter may, for example,be disposed in a mobile unit, terminal device, user equipment, or thelike to transmit information signals on an uplink radio channel.Regardless of the particular type of communication system in which theseexemplary embodiments are presented, the transmit device will typicallyinclude the components illustrated generally in FIG. 6.

Therein, the transmitter includes a plurality of physical transmitantennas 602 in this example numbering four, although more or fewer thanfour transmit antennas can be used. The physical transmit antennas 602are connected to a processor 606 via transmit (TX) chain elements 604which can include one or more of filters, power amplifiers and the like,as will be appreciated by those skilled in the art. Processor(s) 606, inconjunction with memory device(s) 608 (and potentially other devices notshown) can operate to perform the transmit processes discussed abovewith respect to FIGS. 3-5, e.g., by way of software stored therein,additional hardware or some combination of software and hardware. Thus,the precoding functionality described above can, for example, beperformed in software by executing computer-readable instructions frommemory device 608 to perform the matrix multiplications described abovewith respect to FIG. 5. Thus, it will be apparent that exemplaryembodiments also relate to software, e.g., program code or instructionswhich are stored on a computer-readable medium and which, when read by acomputer, processor or the like, perform certain steps associated withtransmitting information signals which are precoded in the mannerdescribed above. An example of such steps is illustrated in theflowchart of FIG. 7.

Therein, at step 700, symbol vectors are precoded by multiplying themwith a first unitary matrix which spreads symbols in the symbol vectorsacross the virtual transmit antennas, a second diagonal matrix whichchanges a phase of the virtual transmit antennas, and a third precodingmatrix which distributes the transmission across the transmit antennas.After precoding the symbol vectors, they can undergo further processingat step 702 to generate information signals. For example, suchadditional signal processing can include mapping precoded symbols toresource blocks to be transmitted via at least one of the transmitantennas and orthogonal frequency division multiplexing (OFDM) theresource blocks, although other processing, e.g., for non-OFDM systems,could alternately be performed downstream of the precoding operation.Then, at step 704, the resultant information signals are transmitted.

Exemplary embodiments also provide for receive side processing ofsignals which have been transmitted using the foregoing exemplaryprecoding embodiments. In systems using common pilots (common referencesymbols (RS)), the receiver needs to be aware of the transmissionstructure in order to be able to properly decode the transmission. LTEis one example of such a system where this transmission mode is usingcommon reference symbols and is thus not transparent to the UE. Thus,all of the involved matrices described above (i.e., W, D and U) need tobe known on the receive side (e.g., at the UE) to be used for equalizingthe channel. For example, the UE may first form the effective channelH_eff=HWDU, where H is a channel estimate obtained from the common RS,equalize the effective channel, e.g. by using a linear filterinv(H_eff^*H_eff)H_eff^*), producing the equalized vector sequence z,which is input to a demodulator, producing soft values of coded bitswhich are finally input to, e.g., a turbo decoder to produce an estimateof the transmitted information bits.

It will be appreciated that there are numerous implementations forreceiving and decoding wirelessly received information symbols and thatthe foregoing is simply one exemplary implementation. The receive sideprocessing according to these exemplary embodiments will essentiallyprovide a mirrored processing to that performed on the transmit side.The receiver will use its knowledge of the precoding performed by thetransmitter to perform its channel estimation/equalization function.Such knowledge on the part of the receiver may be predefined a priori orit may be passed on to the receiver explicitly as part of thetransmitted information.

Thus, an exemplary receiver 800 for receiving and processing informationsignals which have been precoded as described above is illustrated inFIG. 8. Therein, one (or more) receive antennas 802 receive theinformation signals which have been precoded during transmit sideprocessing. After passing through one or more receive (RX) chainprocessing elements 804 (e.g., filters, amplifiers or the like),processor(s) 806 will process the received information signals toextract the information contained therein, e.g., in conjunction withprocessing software stored on memory device(s) 808, by using itsknowledge of the precoding performed on those information signals tocalculate a channel estimate used in subsequent receive side processing.For example, as shown in the flowchart of FIG. 9, a method forequalizing received information signals includes the steps of forming achannel estimate associated with the received information signals bymultiplying an initial channel estimate with a plurality of matrices,the plurality of matrices including a first column subset of a unitarymatrix, a second diagonal matrix, and a third precoding matrix at step900, and equalizing the information signals using the formed channelestimate at step 902.

The foregoing description of exemplary embodiments provides illustrationand description, but it is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Modifications and variationsare possible in light of the above teachings or may be acquired frompractice of the invention. For example, the exemplary embodiments alsoinclude W_(N) _(T) _(×l) and U_(l×r) matrices of more general form and,potentially, also a more general form of CDD matrix, e.g., not limitedto a diagonal matrix. The following claims and their equivalents definethe scope of the invention.

1. A method for transmitting information signals having a plurality ofsymbol vectors associated therewith on a radio channel comprising:precoding said symbol vectors by multiplying said symbol vectors with: afirst column subset of a unitary matrix which spreads symbols in saidsymbol vectors across all virtual transmit antennas, a second diagonalmatrix which changes a phase of said virtual transmit antennas, and athird precoding matrix which distributes transmit energy across physicaltransmit antennas, wherein said symbol vectors are first multiplied bysaid first column subset of unitary matrix, next multiplied by saidsecond diagonal matrix and then multiplied by said third precodingmatrix, further processing said precoded symbol vectors to generate saidinformation signals, and transmitting said information signals.
 2. Themethod according to claim 1, wherein said physical transmit antennas areantenna ports.
 3. The method according to claim 1, wherein whentransmitting using r layers, said third precoding matrix has l columns,said second diagonal matrix has l rows and l columns, said first columnsubset of unitary matrix has l rows and r columns, and said symbolvectors have r elements.
 4. The method according to claim 1, whereinwhen transmitting using r layers, said third precoding matrix has rcolumns, said second diagonal matrix has r rows and r columns, saidfirst column subset of unitary matrix is a unitary matrix having r rowsand r columns, and said symbol vectors have r elements.
 5. The methodaccording to claim 1, wherein said step of further processing furthercomprises: mapping precoded symbols to resource blocks to be transmittedvia at least one of said transmit antennas; and distributing saidresource blocks over the resource element grid of an orthogonalfrequency division multiplexing (OFDM) type of transmission.
 6. Themethod according to claim 1, wherein phase shifts induced by said seconddiagonal matrix are varied with respect to a parameter that is afunction of a position of the resource element used for transmitting aparticular symbol vector.
 7. The method according to claim 6, whereinsaid parameter is a subcarrier index.
 8. The method according to claim6, wherein said parameter is a data resource element index.
 9. Themethod according to claim 1, wherein said first column subset of unitarymatrix and said second diagonal matrix together exhibit the samestructure as cyclic delay diversity (CDD) for spatial multiplexing whenrepresented in the frequency domain.
 10. The method according to claim1, wherein said third precoding matrix is performing channel dependentprecoding.
 11. A transmitter for transmitting information signals havinga plurality of symbol vectors associated therewith on a radio channelcomprising: a plurality of physical transmit antennas; a processor forprecoding said symbol vectors by multiplying said symbol vectors with: afirst column subset of a unitary matrix which spreads symbols in saidsymbol vectors across all virtual transmit antennas, a second diagonalmatrix which changes a phase of said virtual transmit antennas, and athird precoding matrix which distributes transmit energy across saidphysical transmit antennas, wherein said symbol vectors are firstmultiplied by said first column subset of unitary matrix, nextmultiplied by said second diagonal matrix and then multiplied by saidthird precoding matrix and for further processing said precoded symbolvectors to generate said information signals; and a transmit chain ofelements for transmitting said information signals.
 12. The transmitteraccording to claim 11, wherein said physical transmit antennas areantenna ports.
 13. The transmitter according to claim 11, wherein whentransmitting using r layers, said third precoding matrix has/columns,said second diagonal matrix has l rows and l columns, said first columnsubset of unitary matrix has l rows and r columns, and said symbolvectors have r elements.
 14. The transmitter according to claim 11,wherein when transmitting using r layers, said third precoding matrixhas r columns, said second diagonal matrix has r rows and r columns,said first column subset of unitary matrix is a unitary matrix having rrows and r columns, and said symbol vectors have r elements.
 15. Thetransmitter according to claim 11, wherein said step of furtherprocessing further comprises: mapping precoded symbols to resourceblocks to be transmitted via at least one of said transmit antennas; anddistributing said resource blocks over the resource element grid of anorthogonal frequency division multiplexing (OFDM) type of transmission.16. The transmitter according to claim 11, wherein phase shifts inducedby said second diagonal matrix are varied with respect to a parameterthat is a function of a position of the resource element used fortransmitting a particular symbol vector.
 17. The transmitter accordingto claim 16, wherein said parameter is a subcarrier index.
 18. Thetransmitter according to claim 16, wherein said parameter is a dataresource element index.
 19. The transmitter according to claim 11,wherein said first column subset of unitary matrix and said seconddiagonal matrix together exhibit the same structure as cyclic delaydiversity (CDD) for spatial multiplexing when represented in thefrequency domain.
 20. The transmitter according to claim 11, whereinsaid third precoding matrix is performing channel dependent precoding.21. A method for equalizing received information signals having aplurality of symbol vectors associated therewith comprising: forming achannel estimate associated with said received information signals bymultiplying an initial channel estimate with a plurality of matrices,said plurality of matrices including: a first column subset of a unitarymatrix, a second diagonal matrix, and a third precoding matrix, andequalizing said information signals using said formed channel estimate,wherein said initial channel estimate is first multiplied by said firstcolumn subset of the unitary matrix, next multiplied by said seconddiagonal matrix and then multiplied by said third precoding matrix. 22.The method according to claim 21, wherein said first column subset ofsaid unitary matrix was used in transmit processing of said receivedinformation signals to spread symbols in said symbol vectors across allvirtual transmit antennas, said second diagonal matrix was used in saidtransmit processing to change a phase of said virtual transmit antennas,and said third precoding matrix was used in said transmit processing todistribute transmit energy across physical transmit antennas.
 23. Themethod according to claim 22, wherein said physical transmit antennasare antenna ports.
 24. The method according to claim 22, wherein whensaid transmit processing was performed using r layers, said thirdprecoding matrix has/columns, said second diagonal matrix has l rows andl columns, said first column subset of said unitary matrix has/rows andr columns, and said symbol vectors have r elements.
 25. The methodaccording to claim 22, wherein when said transmit processing wasperformed using r layers, said third precoding matrix has r columns,said second diagonal matrix has r rows and r columns, said first columnsubset of unitary matrix is a unitary matrix having r rows and rcolumns, and said symbol vectors have r elements.
 26. The methodaccording to claim 21, wherein said first column subset of said unitarymatrix and said second diagonal matrix together exhibit the samestructure as cyclic delay diversity (CDD) for spatial multiplexing whenrepresented in the frequency domain.
 27. The method according to claim21, wherein said third precoding matrix is performing channel dependentprecoding.
 28. A processor for forming a channel estimate associatedwith received information signals by multiplying an initial channelestimate with a plurality of matrices, said plurality of matricesincluding: a first column subset of a unitary matrix, a second diagonalmatrix, and a third precoding matrix, and wherein said processor usessaid formed channel estimate to equalize said received informationsignals, and wherein said initial channel estimate is first multipliedby said first column subset of unitary matrix, next multiplied by saidsecond diagonal matrix and then multiplied by said third precodingmatrix.
 29. The processor according to claim 28, wherein said firstcolumn subset of said unitary matrix was used in transmit processing ofsaid received information signals to spread symbols in said symbolvectors across all virtual transmit antennas, said second diagonalmatrix was used in said transmit processing to change a phase of saidvirtual transmit antennas, and said third precoding matrix was used insaid transmit processing to distribute transmit energy across physicaltransmit antennas.
 30. The processor according to claim 29, wherein saidphysical transmit antennas are antenna ports.
 31. The processoraccording to claim 29, wherein when said transmit processing wasperformed using r layers, said third precoding matrix has/columns, saidsecond diagonal matrix has l rows and l columns, said first columnsubset of said unitary matrix has/rows and r columns, and said symbolvectors have r elements.
 32. The processor according to claim 29,wherein when said transmit processing was performed using r layers, saidthird precoding matrix has r columns, said second diagonal matrix has rrows and r columns, said first column subset of unitary matrix is aunitary matrix having r rows and r columns, and said symbol vectors haver elements.
 33. The processor according to claim 28, wherein said firstcolumn subset of said unitary matrix and said second diagonal matrixtogether exhibit the same structure as cyclic delay diversity (CDD) forspatial multiplexing when represented in the frequency domain.
 34. Theprocessor according to claim 28, wherein said third precoding matrix isperforming channel dependent precoding.
 35. A transmitter fortransmitting information signals having a plurality of symbol vectorsassociated therewith on a radio channel comprising: a plurality ofphysical transmit antennas; a processor for precoding said symbolvectors by multiplying said symbol vectors with: a first column subsetof a unitary matrix which spreads symbols in said symbol vectors acrossall virtual transmit antennas, a second diagonal matrix which changes aphase of said virtual transmit antennas, and a third precoding matrixwhich distributes transmit energy across said physical transmitantennas, and for further processing said precoded symbol vectors togenerate said information signals by mapping precoded symbols toresource blocks to be transmitted via at least one of said transmitantennas and distributing said resource blocks over the resource elementgrid of an orthogonal frequency division multiplexing (OFDM) type oftransmission; and a transmit chain of elements for transmitting saidinformation signals, wherein said symbol vectors are first multiplied bysaid first column subset of unitary matrix, next multiplied by saidsecond diagonal matrix and then multiplied by said third precodingmatrix.